Ok, this gets curioser and curiouser.
I normally export my PDFs to a network location mounted via OES (the old Novell network directory and file server). When I export there, it gives the error as above. However, if I export to a local directory (c:\temp\file.pdf), they usually export just fine. There's no change to the project between the two exports; it's literally one right after the other.
Opening a PDF (or JPG or SVG) exported to the network location in Notepad++ reveals a host (thousands, sometimes more than 10,000) of NULL characters at the end of the file after the %%EOF line. PDFs exported to a local directory do not have these NULL characters, nor do ArcMap 10.4 network-exported PDFs. I'm waiting to chat with IT to see if they have any inkling of why they are being added to the end of the PDFs.
If I delete the NULLs from the network-exported file, it will open in Acrobat. However, when I close the document it will ask if I want to save my changes. If I do so, it then passes the PDF 1.7 check. The locally-exported PDFs fail the 1.7 check as above but still open fine (and don't ask to save changes at the end).
I am having a hangup on one very complex layout (36"x48", tens of thousands of features, lots of transparency) that will open in Acrobat (locally-exported) but immediately freezes whenever you try to zoom or pan. I'm still trying track this one down.
Would someone mind running the 1.7 check (https://www.pdf-online.com/osa/validate.aspx) on one of their PDFs exported from Pro to see if they get the same errors as above? It would be interesting to know if everyone gets the 1.7 errors or just me.